1. Field of the Invention
The present invention is related to an organic electroluminescent element such as for information display terminals.
2. Description of the Related Art
An organic electroluminescent element has an organic luminous layer comprising organic luminescent material between two opposing electrodes. By means of applying electric current to the organic luminous layer between both electrodes, the organic luminous layer emits light. Organic luminescent layer thickness is important so that the organic luminous layer emits light efficiently. Organic luminescent layer thickness has to be about 100 nm. Even more particularly, for example, it is necessary to form a pattern of R (red), G (green), and B (blue) with high accuracy to make this a display panel.
Low molecular materials and polymeric materials are examples of organic luminescent materials that can be used to form the organic luminous layer. Generally, as for low molecular materials, thin film can be formed by vacuum evaporation. A mask with minute patterns is used at this time, and patterns are formed. In a manufacturing method with the use of vacuum processing such as vacuum evaporation, upsizing of a substrate can diminish the accuracy of the patterning.
Thus the following method has been tried recently. Polymer organic luminescent material is dissolved in a solvent, and ink is made. This ink is used, and thin film is formed by a wet coating method. As wet coating methods to form thin film, spin coating method, bar coat method, extrusion coat method and dip coat method are exemplified.
It is difficult to form high minute pattern by a wet coating. In addition, it is difficult to paint so that three colors of the pattern of RGB are separated.
As for the formation method of thin film by a printing method, a separated pattern is formed more easily. Therefore, formation method of thin film by a printing method is more effective.
Among various printing methods, ink jet process (Japanese Patent Laid-Open No. 10-12377 Official Gazette), offset printing (Japanese Patent Laid-Open No. 2001-93668 Official Gazette) and relief printing (Japanese Patent Laid-Open No. 2001-155858 Official Gazette) are proposed.
Most of an organic electroluminescent element comprising plural picture elements includes a substrate and a pattern shaped first electrode. A partition wall is formed between the pattern shaped first electrodes.
The two main purposes of forming a partition wall are described below.
The first purpose is described below. Because a partition wall covers a burr of an edge of a pattern-formed first electrode, a short circuit between a first electrode and a second electrode is prevented. A generation process of burr is explained below. A conductive film for a first electrode is formed on the entire surface of substrate. Thereafter, patterning of a photosensitive material is performed by photo-lithography method. Then the necessary part of a conductive film is masked. Then subsequently a first electrode is formed by etching of an exposed conductive film. In this case, a part (burr) of which unevenness is larger than that of a normal part can be generated at an edge of a pattern-formed first electrode. When an edge of a first electrode has a burr, even if an organic luminescent medium layer including an organic luminescent layer is formed on a first electrode, an organic luminescent medium layer cannot coat the projection part. Therefore when an organic electroluminescent element was made by forming a second electrode on the organic luminescent medium layer, short-circuit between a first electrode and a second electrode occurs. Therefore, an edge of a first electrode is covered by a partition wall.
The second purpose is described below.
Organic luminescent layers including organic luminescent materials which can emit lights of different colors are respectively formed in adjacent light emitting areas. A partition wall is formed to prevent color mixture in this case. When organic luminescent inks of organic luminescent materials which can emit lights of colors such as red, green and blue are dissolved in solvent are used, and organic luminescent layers are formed by printing methods, the concentrations of organic luminescent materials are several percent at most. The concentration of organic luminescent material is usually around 1%, and organic luminescent ink is low in viscosity. When a partition wall is not formed on substrate, ink spreads around a first electrode without ink staying only on a first electrode. Therefore, an organic luminescent ink overflows to an adjacent pixel (a light emitting area). Therefore, color mixture of organic luminescent layers occurs. Thus, it is necessary to form a partition wall between different luminescent color picture elements to prevent color mixture by infiltration of organic luminescent ink to adjacent pixel.
A cross-sectional drawing of a conventional organic electroluminescent element is shown in FIG. 1A. In addition, FIG. 1B is extended figure of the vicinity of a partition wall shown in FIG. 1A. Hole transport layer 3 is formed on first electrode 2 on substrate 1. Red (R) organic luminescent layer 41, green (G) organic luminescent layer 42 and blue (B) organic luminescent layer 43 is formed on hole transport layer 3 respectively. And second electrode 5 is formed on an organic luminescent layer (41, 42 and 43). Light emitting area L is a region sectioned by partition wall 7.
When an organic luminescent layer is formed on a substrate with a partition wall by organic luminescent ink, organic luminescent layer thickness near a partition wall tends to be thick as shown in FIG. 1B. When thick organic luminescent layer is formed in light emitting area L, and thickness of an organic luminescent layer fluctuates in a picture element, luminescent unevenness in a picture element of an organic electroluminescent element occurs.
In addition, an organic luminescent medium layer may include plural functional layers. Among these functional layers, there is a layer which can be formed by the same material for all picture elements. As such a layer, a hole transport layer is exemplified. About a hole transport layer, it is not necessary to perform picture element-shaped patterning. Hole transport material ink is applied by coating methods such as a spin coat method. In this way a hole transport layer is formed on a whole area of a substrate. In this case a similar problem occurs. In other words ink is applied to whole area of substrate to coat all picture elements (a light emitting area). In this case, the ink layer tends to be thick near a partition wall. Thickness of a hole transport layer fluctuates in a picture element. Therefore, luminescent unevenness in a picture element occurs.
In addition, when an organic electroluminescent element comprising plural picture elements is manufactured, organic luminescent inks which emit lights of different colors respectively are applied to plural adjacent picture elements. For this case, non-predetermined organic luminescent ink overflows to adjacent pixel. In other words color mixture occurs. To solve this problem, ink-repellent material is usually added in a partition wall. However, ink-repellent material bleeds from a partition wall to a light emitting area (a picture element) when ink-repellent material is used. An organic luminescent layer and a hole transport layer may not be formed partially in a light emitting area due to this phenomenon.
In these drawings, 1 is a substrate; 2 is a first electrode; 3 is a hole transport layer; 41 is a red (R) organic luminescent layer; 42 is a green (G) organic luminescent layer; 43 is a blue (B) organic luminescent layer; 5 is a second electrode; 6 is an organic luminescent medium layer; 7, 7x and 7y each are a partition wall; 8 is an adhesive: 9 is a sealing medium; 111 is a support medium; 112 is an active layer; 113 is a gate insulator; 114 is a gate electrode; 115 is an interlayer dielectric; 116 is a drain electrode; 117 is a planarizing layer; 118 is a contact hole; 119 is a data line; 120 is a thin film transistor; 10 is an ink tank; 12 is an ink chamber; 14 is an anilox roll; 14a is ink; 16 is a relief printing plate; 18 is a printing cylinder; 20 is a stage; 24 is a substrate; L is a light emitting area; N is a non-light emitting area; S1 is an overlapping width of a partition wall and an organic luminescent layer; and S2 is a overlapping width of partition wall and a hole transport layer.